This invention relates to telecommunications equipment, and more particularly to a subscriber identification memory card for such equipment, of the type having an advice of charge counter indicating the amount corresponding to the communications effected with the card, memory means including a file indicating the maximum value of the advice of charge counter and at least one file indicating at least one telephone number free of charge, means for comparing the amount indicated by the advice of charge counter with the maximum amount of the advice of charge counter, and means for blocking calls according to the result supplied by the comparison means.
The invention further relates to a subscriber identification method intended to be implemented by the memory card, of the type including the steps of comparing the amount indicated by an advice of charge counter, corresponding to the communications effected with a SIM card, with a maximum amount stored on the card, and blocking calls made from the card if the metered amount is greater than or equal to the maximum amount.
Mobile subscriber identification cards are used particularly in mobile equipment of the GSM type and enable a subscriber to identify himself vis-a-vis the mobile communication network. In the GSM environment, they are usually called SIM (Subscriber Identity Module) cards.
Recent SIM cards comprise functions whereby the cost of a call can be indicated in currency selected by the user. This service is known as Advice of Charge, abbreviated as AOC. A counter in the card, generally called the ACM (Accumulated Call Meter), adds up the total amount involved in the current call and in all preceding calls. The ACM generally takes the form of an EF.sub.ACM file which can be incremented by the microprocessor of the SIM card. An ACMmax (ACM Maximum Value) field stored in an EF.sub.ACMmax file on the card indicates the maximum amount of the ACM. The ACM is active only when ACMmax is other than zero; ACMmax may be changed by the user for security purposes, or by the network operator in the case of a prepayment network.
If ACMmax is valid, i.e., other than zero, all calls are blocked as soon as the ACM becomes equal to or greater than ACMmax, possibly after a predetermined interval CAI. The ACM is then reset with the new value at the end of the call; this value may be greater than ACMmax. A message on the display of the subscriber's mobile terminal usually indicates to the user why calls are blocked. All attempts to make calls from the mobile terminal or to receive calls are then blocked until the ACM has been reset. A method of this type is described in International Publication No. WO95/28062, for example.
The AOC function may be used as a security measure by a traveling subscriber to limit the possibilities of use of his SIM card in case of theft, for instance. In this case, the valve of ACMmax, can be changed and the ACM counter reset by the subscriber by entering an additional personal identification number PIN2. The AOC is also used for prepaid smart cards. In this case, the value of ACMmax is made equal to the sales value of the card or to the amount loaded on the card; the ACM is incremented at the time of use, with one incrementation unit corresponding to a predetermined amount in the subscriber's currency. The PIN2 is unknown to the mobile subscriber in the case of prepayment cards.